It is essential to control the supply of steam to steam turbines in accordance with operating conditions. For example, when the load on a turbine changes, it is necessary to modulate the flow of steam to the turbine. In the event the load on a generator is lost or if unacceptable vibrations or thrust loads occur, it is also necessary to quickly, virtually instantaneously, shut down the flow of steam to the turbine. Otherwise, the turbine will quickly obtain an overspeed condition. Steam supply valves are, of course, used to modulate and instantaneously stop the supply of steam to the turbine in response to changes in load on the turbine or a system upset requiring shutdown.
Electrical hydraulic control systems have been developed and provided on modern steam turbine units. However, there are a large number of steam turbines currently in the field operating with mechanical/hydraulic control systems. It has been found desirable to convert the older mechanical/hydraulic control (MHC) systems to the more modern electrical/hydraulic control (EHC) systems. MHC-to-EHC control conversions have been previously supplied on small industrial steam turbines. However, in such conversions, the EHC high pressure actuators were retrofitted and were controlled only by a servo valve which did not have an overriding fast closing capability. In mechanical/hydraulic control systems, there is typically provided an MHC servo enclosure in which a control link within the enclosure is operated by various mechanical devices providing input to the link within the enclosure. For example, mechanically operated speed control governors in MHC systems provide input to the control link within the servo enclosure to control the position of the link. The link, in turn, is coupled through various devices through a valve actuator lever which would actuate the valve to modulate or stop the flow of steam to the turbine.
Accordingly, it is desirable to eliminate the mechanical/hydraulic input to the control link and retrofit existing steam turbines having older MHC systems with EHC systems. While standard EHC valve actuators are available, they are difficult and costly to retrofit as replacements for existing MHC valve actuators. However, because it is also expensive to remove all of the mechanical and hydraulic linkage between the control link and the steam valve actuators, it has been found desirable to essentially replace the mechanical/hydraulic input to the control link in the servo enclosure with an electrical/hydraulic input and use the existing linkage between the control link and the valve actuators to modulate or stop the flow of steam through the steam valves to the turbine rather than replace the entire existing mechanical/hydraulic system.